The present invention relates to a new use of metal-ligand complexes for water disinfection and a new method for producing disinfected water, i.e. water which does not contain infectious bacteria or transmit diseases to humans, for drinking and bathing by using know biocidal capabilities of low-concentration metallic species and chemical reactions of metallic species with binding agents to yield stable complexes unaffected by the chemistry of the water being treated for infectious bacteria.
Disinfected water is one of the most important necessities, if not the most important, for survival. The presence of disease-causing bacteria in natural water can render a water supply unusable, and is therefore a very important water quality parameter.
Disease-causing bacteria or pathogens are organisms capable of infecting or transmitting diseases to humans. Pathogens include species of bacteria, viruses, protozoa, and helminths. Cholera and Typhoid, for example, are transmitted by the bacteria Vibrio cholera and Salmonella typhosa respectively. Another common pathogenic bacteria in drinking water is Escherichia coli, or E. coli.
Current technologies utilize chlorine and ammonia, ozone, ultraviolet light, or hydrogen peroxide to treat non-potable water supplies. Application of these disinfection technologies can, however, yield toxic by-products, or require bulky systems or external power supplies.
Metallic ions (e.g. copper, silver, zinc, and nickel) are known biocides and have been used to treat lakes, and also as biofouling agents in paint and coatings. One complication in using these metals as biocides, however, is the affinity of the metal ions to react with naturally occurring anions present in fresh or natural water. Anions, such as carbonates, phosphates, sulfates, and chlorides form insoluble salts and can precipitate metallic cations out of solution, thereby eliminating the detoxification power of the biocidal metal. The major chemical species in natural fresh water are carbonate (CO.sub.3.sup.2-), iron (Fe.sup.2+), and calcium (Ca.sup.2+). Copper carbonate is insoluble in water, and hence, cupric ions react with carbonate ions and precipitate out of solution. Cations found in natural water, such as calcium and iron, can also limit the effectiveness of a particular complexing agent to bind with a more desired cation such as copper, zinc, or silver. The calcium and iron cations will form coordination bonds with ligands forming complexes, thereby displacing the desired biocide metal into solution and subsequently precipitate with naturally occurring anions in solution.
As used herein, a complex means an assembly of one or more central metal atoms such as Cu.sup.2+, Ag.sup.1+, Ni.sup.2+, or Zn.sup.2+, formed through coordination bonds with ligands and having a net neutral, positive, or negative charge. Ligand or complexing agent refers to atoms or groups of atoms, defined as coordinating atoms, which form coordination bonds to another atom, defined as the central or nuclear atom. Complexing agents, or ligands, can be monodentate, bidentate, tridentate, or tetradentate, for example, the prefix to dentate referring to the number of potential coordination atoms present on the ligand. Examples of ligands, or complexing agents, which will form complexes with metal ions include citric acid, EDTA (ethylene-diamine-tetra-acetic acid), and salicylic acid. Metals means the transition metals of the periodic chart in addition to Group IA and IIA metals.
With regard to the biocidal effectiveness of copper, it is theorized that bacteria are destroyed by the reaction of the metal with essential amino acids. In this regard, see Kemmer, F., The NALCO Water Handbook (1988). Copper salts, primarily copper sulfate, is added to water supply reservoirs, retention ponds, and canals to control algae growth. As noted above, copper salts are also added to paint as an antifouling agent as described in French, M. S. and L. V. Evans, The effects of copper and zinc on growth of the fouling diatoms Amphora and Amphiprora. Biofouling, Vol. 1, No. 1, pp. 3-18, 1988; and Mittelman, M. W. and D. C. White, Biofilm ecology of bioluminescent bacteria. Report, Tennessee University, 1992.
Studies performed by San Diego State University have shown the effectiveness of copper in preventing microbial activities in water sampling devices. See Lewis, D. L., A. P. Simons, W. B. Moore, and D. K. Gattie, Treating soil solution samplers to prevent microbial removal of analytes. Applied environmental microbiology, Vol. 58, No. 1, pp. 1-5, 1992. Silver in its ionic form is also often used for disinfection as described in Peavy, H., D. Rowe, and G. Tchobanoglous, Water Resources and Environmental Engineering. McGraw-Hill Book Company, New York, 1985; and Pontius, Frederick, Water Quality and Treatment. McGraw-Hill Book Company, New York, 1990. Other metallic species such as tin, zinc, and nickel have also been suggested as biocides.
The precipitation of metal species from solution as a result of naturally occurring anions in water supplies is illustrated by Bertine, K. K., and M. F. Mendeck, Industrialization of New Haven, Connecticut, as recorded in reservoir sediments. Environmental Science & Technology, Vol. 12, No. 2, pp. 201-207, 1978. Two lakes in Connecticut showed high levels of copper in the form of copper carbonate in the sediments due to excessive usage of copper sulfate. Other species such as sulfate (SO.sub.4.sup.2-), chloride (Cl.sup.1-), and nitrate (NO.sub.3.sup.1-) may also be present in natural water, but their solubility product with respect to copper is relatively higher than that of copper carbonate or copper phosphate and hence are relatively insignificant. However, chloride will react with silver ions to produce silver chloride which is insoluble causing silver to be removed from the solution.
To maintain effective levels of metallic biocides in solution, a suitable complexing agent, or ligand, must be used which (1) effectively shields the metallic ion from anions present in solution (i.e., CO.sub.3.sup.2-, SO.sub.4.sup.2-, NO.sub.3.sup.1-, Cl.sup.1-, PO.sub.4.sup.2+) while (2) keeping the metal available for biocidal action. We have discovered that coordination-compound chemistry is the best solution. Complexing agents, or ligands, can be monodentate, bidentate, tridentate, tetradentate, etc. Typical examples are OH.sup.1-, NH.sub.3, PO.sub.4.sup.3-, Cl.sup.1-, --COO.sup.2- as monodentate ligands; glycinate (NH.sub.2 CH.sub.2 COO.sup.-), oxalate (.sup.- OOC--COO.sup.-), salicylate (.sup.- OC.sub.6 H.sub.4 COO.sup.-) as bidentate ligands; citrate (H.sub.2 C--COO--OHCCOO--H.sub.2 C--COO) as a tridentate ligand; and EDTA, (.sup.- O.sub.2 CCH.sub.2).sub.2 --NCH.sub.2 CH.sub.2 N--(CH.sub.2 CO.sub.2.sup.-).sub.2 as a hexadentate ligand. Coordination complexes have also naturally developed in biological systems. For example, magnesium is complexed with chlorophyll in plants, and iron is complexed by the porphyrin group of hemoglobin.
We have recognized two important parameters which must be considered when choosing a chelating agent, namely (1) the competition of H.sup.+ and other cations with the primary biocidal metal ion for the free pairs of electrons of the coordination atom; and (2) the constant of formation of the metal-ligand complex. In accordance with our invention, the optimum ligand on a chemical basis is selected based on the minimum pH at which the full deprotonated dentate is formed, the number of free pairs of electrons per ligand, and the constant of formation of the ligand with the desired metal ions versus other cations (e.g., Ca.sup.2+, Fe.sup.2+) present in natural water. Another heavily weighted factor in the overall feasibility determination of the final complex formulation will include the human health effect. Copper, silver, nickel, and zinc are regulated by the Environmental Protection Agency, and the water quality standards are 1.0, 0.05, 0.10, and 5.0 mg/L, respectively. The disinfection processes and biocide solutions of the present invention described herein by example meet current EPA requirements for potable drinking water.
It is, therefore, an object of the present invention to substantially improve disinfectant technology by using pre-mixed solutions of metal-ligand complexes as a stable disinfectant for water containing calcium, iron, carbonates, chlorides, nitrates, and sulfates. Although complexing agents such as ETDA have been used for cleaning applications and food preservation, and complexed metals have been used as biofouling additives for paint and coatings, the use of metal-ligand complexes as an additive for water disinfection and subsequent use for consumption or cleaning has not been recognized.
It is another object of the present invention to provide a method which has both military and commercial sector applications, such as remote field use or camping water supply, third world country assistance, and domestic and foreign disaster relief efforts.
It is still a further object of the present invention to provide a water disinfecting method which can be scaled up for use in municipal water treatment facilities and also in countries with scarce and/or unsafe drinking water.
These and other objects have been achieved by use of non-toxic complexing agents that bind to the metallic ions, providing a "shield" from reactive anions while retaining the metal's biocidal capacity to kill pathogenic bacteria and microorganisms.